How does the Orthodox church justify the practices of feast days in light of these passages? Or rather how does the church interpret this differently than I am doing?
We dont have to justify it since the Apostles kept days, feasts and fasts. We have no right to decide they really had no idea what they were doing so we're going to correct the Apostles.
Read the book of Acts. Throughout the entire book we read of Apostles keeping hours of prayer, (the same hours the Orthodox Church keeps to this day...the 1st, 3rd, 6th and 9th hours)
We even read in 2 different places (at least) where St. Paul made specific efforts to return to the Mother Church of Jerusalem to keep specific Church FEASTS.
"But Paul took leave of them saying I must by all means keep the comming feast in Jerusalem" Acts 18:21
Another time we read, "For Paul had decided to sail past Ephesus so that he would not have to spend time in Asia for he was urrying to be at Jerusalem, if possible, on the Day of Pentecost. -Acts 20:18
There are other instances in the book of Acts as well, but these are the ones in reference to St. Paul. Certainly if Paul was keeping feasts, even going so far as to travel to the Church of Jerusalem to keep them, he obviously didn't mean that keeping feasts is somehow wrong.
He kept fasts and feasts, the other Apostles did as well, and if they did it then so should we.
Does that help any?